The long-range objective of this project is to determine the changes in regional cerebral blood flow and in cerebral vascular resistance in anesthetized cats following experimental concussive brain injury. Measurements of blood flow will be made with the injection of radioactive microspheres and with a method based on simultaneous measurement of cerebral arterial diameter and flow velocity with an ultrasonic flowmeter. The responses of cerebral blood flow and cerebral vascular resistance to arterial hypercapnia, arterial hypocapnia, arterial hypoxia, decreases in arterial blood pressure and increases in intracranial pressure will be explored. These interventions occur frequently in patients with brain injuries. It is expected that the studies will determine the extent, location, time course and mechanism of the abnormalities in the regulation of the cerebral circulation following experimental brain injury.